The present invention relates to a method and an apparatus for the production of shaped articles.
Production of shaped articles which are formed in a mold tool in a manner which avoids sink marks or other surface defects has long been sought by those in the art. In such a molding process a build up of dwell pressure is necessary in many cases, in particular with the so-called RIM (reaction injection molding) method. There have been attempts to produce this dwell pressure by charging the reaction mixture exceedingly strongly with gas. However, this measure only produces the desired effect of avoiding surface defects with shaped articles having wall thicknesses greater than about 2.5 mm. With long flow paths, however, and during passage through mold cavity portions of reduced thickness, the back pressure produced causes undesirable partial dissolution of the gases in the reaction mixture. As a result, the dissolved gas is no longer available for pressure compensation during the very rapid solidification of the reaction mixture and the reaction shrinkage occurring.
If the admission rate is too slow or the filling time too long, the viscosity begins to increase during introduction of the mixture due to the initiation of the solidification reaction with a build up of pressure preferably in the gate region. A uniform build up of dwell pressure of, for example, 70 bar or higher during the curing process cannot be achieved with this type of dwell pressure control.
There have also been attempts to adopt suitable over-filling techniques for cell-forming reaction mixtures. However, the shaped article material has an undesirably high density, and a considerable quantity of material is wasted due to the over-filling. Less rigid mold tools and closure units have been used with this method in order to utilize the swelling thereof. Strong, irregular expulsion, the low closing pressure and difficultly reproducible dimensional accuracy prevented the mass production of high quality shaped articles. Due to the low pressure level attainable, the formation of bubbles on the surface of the shaped article could not be reliably prevented in the region of sharp-edged transitions.
There have also been attempts (DE-OS No. 28 29 016 corresponding to U.S. Pat. Ser. No. 4,256,679) to use a storage piston which is arranged between the mixer head and the actual mold cavity. In this process, the mold cavity is enlarged by the storage volume before introduction of the mixture and pressure is then applied, creating an elevated pressure. Markings on the shaped article cannot be avoided. This dwell pressure control is also impaired by penetration and solidification of the reaction mixture in the sealing gap between piston and storage cylinder. This technique cannot be adopted for geometrically complicated or for long shaped articles because the propagation of pressure over the entire mold cavity is not ensured due to progressive solidification.
There have also been attempts (EP-PS No. 24 610 corresponding to U.S. Pat. Ser. No. 4,314,955) to use mold tools with elastic internal wall regions which regions can be charged from the rear. The disadvantage of these mold tools resides in the fact that uniform heating and cooling of the elastic wall regions causes problems. A further draw-back lies in the fact that the dimensional accuracy of the shaped articles is not ensured.